Gli scienziati sanno bene che la materia scura è là fuori e rappresenta non solo lo ‘scheletro cosmico’ sul quale sono distribuite le strutture che vediamo oggi sotto forma di galassie e ammassi di galassie ma essa gestisce, per così dire, la dinamica stessa dell’intero Universo. La domanda è: cos’è la materia scura? Qual è la natura delle particelle che la compongono? Queste, ed altre, domande rimangono ancora senza risposte mentre i numerosi esperimenti sparsi sul globo tentano di far luce a questo enigma della moderna cosmologia.
La nostra attuale comprensione sulla composizione dell’Universo è basata sul fatto che la maggior parte della sua massa consiste di materia scura, lo ‘scheletro cosmico’ su cui sono distribuite le galassie e gli ammassi di galassie. Tra le sue proprietà, ricordiamo che la materia scura è fredda, massiccia, non ha colore né carica elettrica e può essere rivelata solamente mediante gli effetti gravitazionali che essa esercita sulla materia ordinaria e sulla radiazione.
In un recente articolo apparso su Physical Review Letters, un gruppo di tre fisici suggerisce che i risultati di un esperimento condotto negli anni ’80 pone dei limiti all’idea secondo la quale i fotoni ‘scuri’ potrebbero essere le particelle candidate per spiegare la natura della materia scura. Gli scienziati, Brian Batell, Rouven Essig e Ze’ev Surujon, fanno notare che esiste già una proposta per realizzare un esperimento, non troppo costoso, che potrebbe essere utilizzato come test per verificare, o smentire, questa ipotesi.
This program is oriented towards WIMP (10th-16th) and axion (17th-21st) dark matter searches, covering also other candidates. The aim is to promote the collaboration and exchange of ideas. The list of invited speakers includes theoretical as well as experimental experts. Main topics of the workshop include: Dark Matter, WIMP, Axion, Astro-particle physics.
Il gruppo di 40 fisici che collaborano all’esperimento Particle and Astrophysical Xenon Detector (PandaX) guidati dalla Shanghai Jiao Tong University e che ha lo scopo di dare la caccia alla materia scura in un laboratorio sotterraneo situato nella parte sud-ovest della Cina, hanno pubblicato sulla rivista Science China Physics, Mechanics & Astronomy i risultati delle prime fasi dell’esperimento. PandaX è il primo esperimento sulla materia scura che viene realizzato in Cina e che utilizza un rivelatore contenente più di cento chili di xeno. Il progetto è stato studiato per monitorare le eventuali collisioni tra i nucleoni dell’atomo di xeno e le cosiddette Weakly Interactive Massive Particles (WIMPs) che sono le principali indiziate per la materia scura. Continua a leggere PandaX-1, i primi risultati delle ricerche sulla materia scura
L’Ufficio di Scienza del Dipartimento di Energia (DOE) degli Stati Uniti e la National Science Foundation (NSF) hanno annunciato in questi giorni il finanziamento di tutta una serie di progetti allo scopo di cercare nuove tracce dell’enigmatica materia scura con una sensibilità molte volte superiore a quella che caratterizza gli attuali esperimenti. Denominati Generation 2 Dark Matter Experiments essi includono l’esperimento LUX-Zeplin (LZ), una collaborazione internazionale istituita nel 2012 che sarà gestita dal DOE presso il Lawrence Berkeley National Laboratory i cui rivelatori saranno installati presso la Sanford Underground Research Facility (SURF) nel South Dakota, e altri due esperimenti. Continua a leggere Al via i nuovi esperimenti sulla materia scura
Dark matter, the mysterious substance estimated to make up approximately more than one-quarter of the mass of the Universe, is crucial to the formation of galaxies, stars and even life but has so far eluded direct observation. At a recent UCLA symposium attended by 190 scientists from around the world, physicists presented several analyses that participants interpreted to imply the existence of a dark matter particle.
Dark matter is widely thought to be a kind of massive elementary particle that interacts weakly with ordinary matter. Physicists refer to these particles as WIMPS, for weakly interacting massive particles, and think they originated from the Big Bang. WIMPs are thought to be streaming constantly through the solar system and the Earth.
Dopo 90 giorni di attesa, gli esperimenti hanno mostrato che LUX (Large Underground Xenon) rappresenta il rivelatore più sensibile per lo studio della materia scura. Nonostante i primi risultati non abbiano permesso di rivelare particelle esotiche essi forniscono alcune indicazioni escludendo alcune ipotesi. Ora i ricercatori si concentreranno per migliorare la sensibilità del rivelatore che sarà impegnato nel 2014 con un esperimento che avrà la durata di 300 giorni.
In its first three months of operation, the Large Underground Xenon (LUX) experiment has proven itself to be the most sensitive dark matter detector in the world, scientists. “LUX is blazing the path to illuminating the nature of dark matter”, Rick Gaitskell, professor of physics at Brown University and co-spokesperson for LUX. The detector’s location, more than a mile underground at the Sanford Underground Research Facility in South Dakota, offers a “supremely quiet” environment to detect the rare, weak interactions between dark matter particles and ordinary matter, Gaitskell said. The first results from the experiment’s initial 90-day run were announced during a seminar at the Sanford Lab in Lead, S.D. “What we’ve done in these first three months of operation is look at how well the detector is performing, and we’re extremely pleased with what we’re seeing”, said Gaitskell, one of the founders of the LUX experiment. “This first run demonstrates a sensitivity that is better than any previous experiment looking to detect dark matter particles directly”.
With LUX’s initial run complete, the team will now make a few adjustments to fine-tune the device’s sensitivity in anticipation of a new 300-day run to begin in 2014.
Dark matter is thought to account for as much as 85 percent of the matter in the Universe. But because it rarely interacts with other forms of matter, it has yet to be detected directly. The leading candidates for dark matter particles are called weakly interacting massive particles (WIMPs). Theory and experimental results suggest that WIMPs could take either a high-mass or low-mass form. In the search for high-mass WIMPs weighing 40 times the mass of a proton, LUX has three times the sensitivity of any other dark matter direct-detection experiment, according to these new results. LUX also has greatly enhanced sensitivity to low-mass WIMPs, and new results suggest that potential detections of low-mass WIMPS by other dark matter experiments were likely the result of background radiation, not dark matter. “There have been a number of dark matter experiments over the last few years that have strongly supported the idea that they’re seeing events in the lowest energy bins of their detectors that could be consistent with the discovery of dark matter”, Gaitskell said. “With the LUX, we have worked very hard to calibrate the performance of the detector in these lowest energy bins, and we’re not seeing any evidence of dark matter particles there”.
In the upcoming 300-day run, the LUX researchers hope either to detect dark matter definitively or to rule out a vast swath of parameter space where it might be found.
“Every day that we run a detector like this we are probing new models of dark matter”, Gaitskell said. “That is extremely important because we don’t yet understand the Universe well enough to know which of the models is actually the correct one. LUX is helping to pin that down.” Though dark matter has not yet been detected directly, scientists are fairly certain that it exists. Without its gravitational influence, galaxies and galaxy clusters would simply fly apart into the vastness of space. But because dark matter does not emit or reflect light, and its interactions with other forms of matter are vanishingly rare, it is exceedingly difficult to spot. “To give some idea of how small the probability of having a dark matter particle interact, imagine firing one dark matter particle into a block of lead”, Gaitskell said. “In order to get a 50-50 chance of the particle interacting with the lead, the block would need to stretch for about 200 light years, this is 50 times farther than the nearest star to the Earth aside from the sun. So it’s an incredibly rare interaction”. Capturing those interactions requires an incredibly sensitive detector. The key part of the LUX is a third of a ton of supercooled xenon in a tank festooned with light sensors, each capable of detecting a single photon at a time. When a particle interacts with the xenon, it creates a tiny flash of light and an ion charge, both of which are picked up by the sensors. To minimize extraneous interactions not due to dark matter, the detector must be shielded from background radiation and cosmic rays. For that reason, the LUX is located 4,850 feet underground, submerged in 71,600 gallons of pure de-ionized water. But even in that fortress of solitude, occasional background interactions still happen. It’s the job of LUX physicists to separate the signal from the noise.
During its initial run, the LUX picked up xenon flashes in the energy region of interest for dark matter at a rate of about one per day. By looking carefully at the nature of each interaction, the researchers can tell which ones are from residual background radiation and which could be due to dark matter.
“Dark matter will interact with the nucleus of xenon atom, while most forms of radioactive background tend to interact with the outer electrons”, Gaitskell explained. “Each of those interactions produces a recoil, either of the nucleus or the electrons. So at the rate of about one a day, we see these interactions and test to see if they are consistent with a nuclear recoil or an electron recoil. So far every event we have seen has looked like a conventional electromagnetic background event”.
But as the detector runs for longer periods, the odds that a dark matter interaction will be captured increase. And the LUX, says Gaitskell, has the sensitivity to catch it.
“LUX is a huge step forward. Within the first few minutes of switching it on, we surpassed the sensitivity of the first dark matter detectors I was involved with 25 years ago,” Gaitskell said. “Within a few days, it surpassed the sensitivity of sum total of all previous dark matter direct search experiments I have ever worked on. This first LUX run is more sensitive than any previous search conducted and now sets us up perfectly for the 300-day run to follow”.
Brown University: First results from LUX dark matter detector
Now that it looks like the hunt for the Higgs boson is over (post), particles of dark matter are at the top of the physics “most wanted” list. Dozens of experiments have been searching for them, but often come up with contradictory results. Theorists from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), a joint SLAC-Stanford Institute, believe they have come up with an algorithm, a mathematical description of how the individual particles behave, that could help narrow the search for these elusive particles, which are thought to make up more than 25 percent of the matter and energy in the Universe.
“It starts with assumptions”, said Yao-Yuan Mao, lead author of a paper published in The Astrophysical Journal that outlines their new search tool. Assumptions are a good starting point when you don’t know where to look. A popular assumption about dark matter is that it’s made up of WIMPs, Weakly Interacting Massive Particles. The “M” in WIMP accounts for gravity’s ability to herd these particles around; the “P” and “I” hint at why they are so hard to detect otherwise. Most dark matter detectors are based on the assumption that, every once in a while, a WIMP must smack into the nucleus of an atom of visible matter, making the nucleus vibrate and releasing a signal. Such disruptions can be detected. But what that disruption looks like and how often it happens depends on yet more assumptions. “How heavy is the dark matter particle? How fast is it moving? Another common assumption that touches on these issues”, said Mao, “is that collections of WIMPs behave as an ideal gas, a collection of particles that hang out together and occasionally bounce off each other. Sometimes a lucky bounce gives a particle more energy, sending it zooming off at a greater speed. How often particles pick up more energy and more speed depends on how much you turn up the heat or put on the pressure. But, as far as scientists can tell, turning up the heat and putting on the pressure doesn’t affect WIMPs. Only gravity does. “The Ideal Gas Law doesn’t describe a system of particles, like dark matter particles, that don’t seem to transfer energy to each other”. This incorrect description can distort the carefully built picture upon which a search for WIMPs is based. In particular, it means predictions of their velocities can be off by a significant amount, but velocities affect what a detector will see. Mao and his colleagues have used simulations to provide new insight into how fast WIMPs are expected to move. WIMPs that move fast enough to reach escape velocity and leave the dark matter halo that surrounds the Milky Way take themselves completely out of the hunt. That reduction in the number of WIMPs affects how often one hits the nucleus of an atom in a detector. The remaining WIMPs must be moving more slowly than escape velocity, which affects how hard they can hit. If they hit a detector whose atoms are too massive, the WIMPs bounce off without a sign, like pebbles scattering off a boulder. So the trick is to build a detector out of materials that are a good match for the particle’s expected mass and speed. As theorist Louis Strigari, another author on the paper, said, “The heavier the WIMP, the more collisions you can detect“. But there is a growing suspicion that WIMPs might be as much as 10 times lighter than previously thought. “If WIMPs do have this low mass“, said Strigari, “the model used to describe their behavior would have significant effects on an experiment’s result“. In fact, Mao, Strigari and Risa Wechsler, a professor at Stanford, are now busy interpreting the results of experiments based on their new description, and they believe it explains some of the conflicting results obtained by such experiments as XENON100 (which uses the fairly heavy element xenon as the material for dark matter to smack into) and the Cryogenic Dark Matter Search II, or CDMS II (which took its readings with detectors made from the much lighter element silicon). KIPAC member Blas Cabrera is a Stanford physics professor and, as a leader of CDMS II, a dark matter hunter from the experimental side. He said that “Theorists have made an important contribution. It really emphasizes that, for light-mass WIMPs, different types of detectors would have different responses. I’m actually hoping we can talk the experimental community into using their model. It’s important to get everyone to agree to use the same parameters so we’re comparing apples to apples instead of apples to oranges“.